There are known in the art several designs for reducing a descending bed of iron ore particles in a moving bed particles reduction reactor countercurrently with an ascending stream of reducing gas, typically comprising hydrogen and carbon monoxide. These furnaces are generally cylindrical and are insulated and refractory lined so that the metallic walls of the furnace vessel withstand the high reduction temperature, on the order of 800.degree. C. to 1100.degree. C., and withstand also the abrasion and pressure of the descending bed of particles.
So that all particles are uniformly reduced and in order to obtain a product of homogenous composition, it is necessary to design the shaft furnace for a mass-flow of particulate solids. The term mass-flow, as used in the art, means that solids move in all regions of the volume of the bed of solids in the vessel. It is also particularly desirable to design the shaft furnace to produce a uniform flow of particles through the reduction section of said furnace. This means that all particles travel in a plug flow, i.e. at the same velocity, and consequently, have the same residence time within said reduction section. Also, great care is exerted for assuring an effectively uniform countercurrent flow of gases through the furnace by proper design of gas inlets and outlets.
Residence time of solids is regulated by a suitable mechanism, located at the bottom of the furnace, which regulates the rate of discharge of solids. This mechanism can be rotary or star-type feeders, vibratory feeders, etc. well known in the art. See as preferred, U.S. pat. No. 4,427,135 issued to one of the present applicants and his co-workers. So that the discharge mechanism is of a practical size, the solids discharge outlet at the bottom of the furnace is of a cross sectional area smaller than the cross sectional area of the reduction section, and for this reason the lower section of the furnace, typically, takes the shape of a downwardly converging cone.
It is known that in order to maintain plug flow in the cylindrical reduction zone, one must have mass flow of particles in the discharge cone, i.e. said particles must flow in all regions of the volume occupied by said particles, including those in contact with the furnace walls. To this end, the angle of the conical section must be selected according to the flow characteristics of said particles, both in respect to each other and in respect to the material and conditions of the internal surface of the conical wall (e.g. temperature, size distribution, roughness of surface, etc.). The angle is chosen so as to avoid bridging to form arches or domes of the particles within the furnace, which would interrupt the gravity flow. This is particularly critical when treating potentially sticky or cohesive particles, for example, hot direct reduced iron (also known as DRI or sponge iron).
It would seem logical that in order to discharge DRI at high temperature, it would be necessary to avoid heat loss which in turn would indicate that heat should not be removed from the conical wall. This would suggest putting the same insulating and refractory material covering on the wall of the conical section as is usually placed on the wall of the cylindrical reduction section of these furnaces in order to minimize heat loss.
However, applicants have recognized that the roughness of the refractory materials impedes the flow of the particles through the discharge cone, requiring a steeper conical wall in order to maintain the necessary mass flow of the particles in the cone. Furthermore, applicants have also discovered that the flow characteristics of DRI change with temperature, giving an increase in apparent friction with a corresponding increase in temperature. This again requires a steeper wall for the discharge cone.
On the other hand, in practice, it would not be feasible to build direct reduction furnaces having steeper conical walls than required to discharge DRI at low temperatures as is presently done, because those furnaces would be too long and costly, or the discharge opening would be too large. Furthermore, the particles would tend to consolidate or stick due to the longer residence times at high temperatures within the furnace, resulting from the longer cone.
In an attempt to solve the above problems, it has been proposed in the past to operate internal mechanisms to promote the flow of solids. This alternate solution is not practical because said mechanisms operate under very severe inside conditions inside the reduction furnace, and also, because they obstruct uniform flow and generate undesirable fines.
Since there is a great interest in discharging DRI at the highest possible temperature suitable for immediate melting or immediate hot briquetting, the need still exists for producing DRI at a temperature above at least 500.degree. C., and preferably, above 700.degree. C.
This invention is directed to solving these contradictory requirements in a novel and counterintuitive fashion that is particularly advantageous and useful in the art.
It is accordingly an object of the present invention to provide a method and apparatus for producing hot DRI in an economical and practical way.
It is another object of the invention to provide a method and apparatus for discharging by gravity hot DRI from a vertical shaft furnace without solids flow problems and with good quality.
It is a further object of the invention to provide a method and apparatus for maintaining the overall shorter size of the lower discharge section of the furnace similar to those with a cold discharge so that the hot DRI particles flow by gravity, and at the same time, permit the average bulk temperature of DRI to be maintained at a level suitable for immediate melting or hot briquetting.